In the manufacture of various items, such as printed circuit (“PC”) boards, it is frequently necessary to apply small amounts of viscous fluid materials, i.e. those with a viscosity greater than fifty centipoise, to substrates. Such materials include, by way of example and not by limitation, general purpose adhesives, solder paste, solder flux, solder mask, grease, oil, encapsulants, potting compounds, epoxies, die attach pastes, silicones, RTV and cyanoacrylates.
As one example, a fabrication process known as flip chip technology has developed, which has multiple processes that require viscous fluid dispensing. For example, a semiconductor die or flip chip is first attached to a PC board via solder balls or pads, and in this process, a viscous solder flux is applied between the flip chip and the PC board. Next, a viscous liquid epoxy is dispensed and allowed to flow and completely cover the underside of the chip. This underfill operation requires that a precise amount of the liquid epoxy be deposited along at least one side edge of the semiconductor chip. As the volume of the epoxy decreases during the curing process, a pseudo-hydrostatic state of stress will be imposed on the solder balls or pads, and this will provide resistance to deformation of the solder balls or pads, and therefore resistance to fracture. The liquid epoxy flows under the chip as a result of capillary action due to the small gap between the underside of the chip and the upper surface of the PC board. Once the underfill operation is complete, it is desirable that enough liquid epoxy be deposited to encapsulate all of the electrical interconnections, so that a fillet is formed along the side edges of the chip. A properly formed fillet ensures that enough epoxy has been deposited to provide maximum mechanical strength of the bond between the chip and the PC board. It is critical to the quality of the underfilling process that the exact amount of epoxy is deposited at exactly the right location. Too little epoxy can result in corrosion and excessive thermal stresses. Too much epoxy can flow beyond the underside of the chip and interfere with other semiconductor devices and interconnections. These parameters must be accurately controlled in the context of manufacturing environments that require high speed productivity.
In another application, a chip is bonded to a PC board. In this application, a pattern of adhesive is deposited on the PC board; and the chip is placed over the adhesive with a downward pressure. The adhesive pattern is designed so that the adhesive flows evenly between the bottom of the chip and the PC board and does not flow out from beneath the chip. Again, in this application, it is important that a precise amount of adhesive be deposited at exact locations on the PC board.
The PC board is often being carried by a conveyor past a viscous material dispenser that is mounted for two axes of motion above the PC board. The moving dispenser is often of the type capable of depositing small dots or droplets of viscous material at desired locations on the PC board. This type of dispenser is commonly referred to as a non-contact jetting dispenser. There are several variables that are often controlled in order to provide a high quality viscous material dispensing process. First, the weight or size of each of the dots is controlled. Known viscous material dispensers have closed loop controls that are designed to hold the dot size constant during the material dispensing process. It is known to control the dispensed weight or dot size by varying the supply pressure of the viscous material, the on-time of a dispensing valve within the dispenser and the stroke length of a valve member of the jetting dispenser. Known control loops have advantages and disadvantages depending on the design of a particular dispenser and the viscous material being dispensed. However, known techniques often require additional components and mechanical structure, such as weigh scales, thereby introducing additional cost, time and reliability issues. Further, known methods often involve the use of calibration procedures, separate from the manufacturing process, which reduces productivity. Therefore, there is a continuing need to provide faster and simpler means for controlling parameters such as dot size, and dispensed fluid volume or weight.
Another important variable that may be controlled in the dispensing process is the total amount of viscous material to be dispensed in a particular cycle. Often the designer of a chip specifies the total amount of viscous material, for example, epoxy in underfilling, or adhesive in bonding, that is to be used in order to provide a desired underfilling or bonding process. In jetting, for example, for a given dot size and dispenser speed, it is known to program a dispenser control so that the dispenser dispenses a proper number of dots to dispense a specified amount of the viscous material in a desired line or pattern at the desired location. Such a system is reasonably effective when the dispensing parameters remain constant. However, such parameters are constantly changing, albeit, often only slightly over the short term. The cumulative effect of such changes can result in an undesirable change in the volume of fluid being dispensed by the dispenser. Therefore, there is also a need for a control system that can detect changes in dispensed amount and make automatic adjustments, so that the desired total amount of viscous material is uniformly dispensed over an entire dispensing cycle.
Current systems presume that the density of the viscous material remains constant. However, the density is constantly changing, resulting in an undesirable variance in the amount of fluid actually dispensed or jetted. It is desirable that the dispensed amount of viscous material has particular mass, volume, and density values, or has an acceptable tolerance of mass, volume, and density values. Having precise values of the density of the dispensed viscous material allows for greater precision in dispensing the desired amount of viscous material onto the substrate. Therefore, there is also a continuing need for precisely obtaining the density of the viscous material as the viscous material is being dispensed, and adjusting the dispensing parameters based on the density or specific gravity.
Generally, there is a need for an improved computer controlled viscous fluid dispensing system that addresses these and other challenges of accurately dispensing small amounts of viscous fluid in high productivity manufacturing processes and the like.